1. Field of the Invention
The present invention relates to a method for efficiently producing water-soluble cellulose ether containing a smaller amount of insoluble fibers.
2. Description of the Related Art
For the preparation of cellulose ether, known is a method of bringing an alkali solution into contact with a highly purified pulp to prepare the corresponding alkali cellulose, and etherifying the resulting alkali cellulose with an etherifying agent. The cellulose ether obtained as a final product becomes water-soluble by properly controlling its degree of substitution. The water-soluble cellulose ether contains a water-insoluble portion which sometimes lowers the light transmittance of the aqueous solution of the cellulose ether or damages the commodity value of the cellulose ether as a contaminant. This insoluble portion appears to be caused by the presence of a low-substituted portion which does not have enough substituents to dissolve in water. One of the reasons why the insoluble portion is present is that an alkali distribution is uneven in the alkali cellulose.
Functions of this alkali include swelling the cellulose therewith to change its crystal structure in the pulp and thereby promoting the penetration of the etherifying agent; catalyzing the etherification reaction with an alkylene oxide; and serving as a reactant for an alkyl halide. A portion of pulp not brought into contact with the aqueous alkali solution is not involved in the reaction and therefore remains as an undissolved portion. Lack of uniformity of the alkali cellulose directly leads to the undissolved portion.
A widely used method for preparing alkali cellulose includes a method comprising the steps of adding an alkali to a powdery pulp, which has been obtained by pulverizing a pulp, in an amount just necessary for the etherification reaction, and mechanically mixing them (Japanese Patent Application Examined Publication No. 60-050801/1985 and Japanese Patent Application Unexamined Publication No. 56-002302/1981). In this method, however, the alkali is not distributed uniformly in the pulp. The pulp not brought into contact with the alkali fails to become cellulose ether and remains in the final product as an unreacted portion, thereby lowering the quality of the cellulose ether.
To avoid such a problem, Japanese Patent Application Unexamined Publication No. 2006-348177 has proposed a method comprising the steps of simultaneously supplying a pulp powder and an aqueous alkali solution to a high speed disperser for continuous contact therebetween to prepare alkali cellulose and reacting the alkali cellulose with an etherifying agent in a reactor. The method still results in cellulose ether containing the pulp not having been contacted with an alkali so that it is not satisfactory with respect to quality of cellulose ether.
Japanese Patent Application Unexamined Publication No. 2007-197682 has proposed a method comprising the steps of continuously bringing pulp into contact with an alkali metal hydroxide solution, draining the obtained contact product with a continuous centrifugal separator to produce alkali cellulose, and reacting the alkali cellulose with an etherifying agent.